Tuesday, January 19, 2016

A
false-color infrared image of a young, star-forming dust cloud with
several embedded cores (identified in red). A new infrared study of 3218
cores in various stages of development has enabled astronomers to
categorize the temperatures, densities, and evolutionary characters of
young stellar nurseries. Credit: ASA/Spitzer and P. Myers

Stars
like the Sun begin their lives as cold, dense cores of dust and gas
that collapse under the influence of gravity until nuclear fusion is
ignited. These cores contain hundreds to thousands of solar-masses of
material and have gas densities about a thousand times greater than
typical interstellar regions (the typical value is about one molecule
per cubic centimeter). How the collapse process occurs in these embryos
in poorly understood, from the number of stars that form to the factors
that determine their ultimate masses, as well as the detailed timescale
for stellar birth. Material, for example, might simply fall freely to
the center of the core, but in most realistic scenarios the infall is
inhibited by pressure from warm gas, turbulent motions, magnetic fields,
or some combination of them.

Astronomers are actively studying these issues by observing young
stars in the process of being born. The dust in these natal cores (or
clumps), however, makes them opaque in the optical, thus requiring
observations at other wavelengths, in particular infrared,
submillimeter, and radio. In the early stages of star formation, an
embryonic star heats the surrounding dust cloud to temperatures between
about ten and thirty degrees kelvin before stellar winds and radiation
blow the material away and expose the newborn star. CfA astronomers
Andres Guzman and Howard Smith, together with their colleagues, have
completed an analysis of 3246 star-forming cores, the largest sample
ever done. The cold cores themselves were discovered with the APEX
submillimeter-wavelength sky survey and then observed in sixteen
submillimeter spectral lines; the spectral information enabled the
astronomers to determine the distance to each core as well as to probe
its chemistry and internal gas motions. The new paper combines these
results with far-infrared measurements taken by Herschel Space
Observatory surveys. The Herschel data allow the scientists to calculate
the dust density, mass, and temperature of each core; the large dataset
then permits useful statistical comparisons between cores with varyious
parameters.

Sources in the sample fall generically into four categories:
quiescent clumps, which have the coldest temperatures (16.8K) and the
least infrared emission, protostellar clumps, which are sources with the
youngest identifiable stellar objects, ionized hydrogen regions, which
are cores within which the stars have ionized some of the surrounding
gas, and "photo-dissociation" cores, the warmest of the set, which have
dust temperatures around 28K, are slightly more evolved and brighter
than the ionized hydrogen cores. Although the groups overlap in their
properties, the large sample enables the scientists to conclude that, on
average, in the quiescent clumps the dust temperature increases towards
the outer regions, whereas the temperatures in protostellar and ionized
hydrogen cores increase towards the inner region, consistent with the
idea that they are being internally heated. The latter also tend to have
dust densities that increase more steeply than the quiescent cores.
This study has also identified a population of particularly cold and
infrared-dark objects that are probably still in the stages of
contraction, or else for some reason have had their star formation
aborted.

The new paper and its catalog are just the beginning: now that
the dust in all these cores has been well characterized, astronomers can
associate chemistry with dust temperature, for example, and study
subgroups that might represent different stellar masses in gestation.